Challenging consults: application of principles of physiology and biochemistry to the bedside. Osmotic diuresis: the importance of counting the number of osmoles excreted.

Polyuria is usually the result of a water diuresis or an osmotic diuresis. Traditionally, the assessment of the extracellular fluid (ECF) volume and the concentration of Na+ in plasma is sufficient to differentiate between the two. We present a case and our approach, which is based on calculations and quantitation of osmoles, to demonstrate the utility of this approach. A patient with diabetes mellitus, human T-cell lymphocyte virus, type 1 (HTLV-1) associated lymphoma, and hypercalcemia presented with marked ECF volume contraction and polyuria. A spot urine osmolality was 567 mOsm/kg H2O in the face of urine output of approximately 6 L/d. The initial diagnosis was an osmotic diuresis. However, a quantitative analysis revealed the enormous number of osmoles could not be accounted for physiologically. Hence, we postulated a water diuresis to be the cause of the polyuria. To confirm this hypothesis, we found that at different times during his hospitalization, the urine specific gravity ranged from 1.005 to 1.022, and urine output varied markedly over 8-h periods. Despite a plasma sodium of 147 mmol/L, the patient did not complain of thirst. Taken together, this suggested the presence of a hypothalamic lesion which caused central diabetes insipidus with variable output of antidiuretic hormone together with a blunted thirst response. Illustration of the utility of a quantitative approach to polyuria is the focus of the discussion.